There are three primary challenges to be met with respect to photonically assisted ADC: quantizing, sampling, and reconstruction. There are fundamental challenges associated with sampling jitter and quantizing resolution. Commonly referred to as the ‘Walden wall’, these fundamental challenges have resulted in slow progress of ADC performance.
One challenge of high-resolution, high-speed photonic ADCs is the quantizer, or quantizing at high fidelity. There are very few physical systems that can provide sufficient (˜1000 levels) resolving power for ten (10) effective number of bits (ENOB), which leads naturally to semiconductor ADCs as a nearly optimal candidate for this operation. Thus, a high-resolution photonic ADC can be viewed as a signal distribution system to permit multiple conventional ADCs to collectively deliver more performance than otherwise possible. These distribution networks can be based on time, space, or frequency, with the most favored approach being frequency-based distribution systems using wavelength division multiplexing (WDM) components, since they provide a relatively low-loss, passive, asynchronous signal routing mechanism. The near-universal adoption of frequency-based distribution naturally favors remaining in frequency space for sampling, too.
A second challenge for the high accuracy photonic ADCs is the performance of the electronic ADC sampler, that is, enhancing Sampler Performance. Aperture jitter can limit its performance, making it unsuitable for the high-speed operations desired. One approach to meet this challenge is to sample the signal before supplying it to the electronic ADC using a higher-performance method. This is often done by imposing the signal on an optical modulator that is sampled with a Mode Locked Laser (MLL) pulse. Such methods must address pulse jitter, amplitude variation, and various nonlinearity effects, and have received considerable study. Alternatively, an electronic sampler can be crafted using an optically-gated RF switch.
An alternative method for addressing sampler limitations is to execute an optical signal processing operation on the information that enhances sampler performance. One of the most popular approaches is the time-stretch ADC (TS-ADC), in which dispersive time spreading of a frequency chirped signal is coupled with a frequency-based signal distribution method to enhance the precision of conventional ADC samplers. A trade-off between spectral efficiency and dispersion bounds the range of this approach, with power spreading, optical loss and amplification providing another constraint. Recent TS-ADC work has focused on compensation of non-ideal dilation, dispersive nonlinearity, and modulation effects.
A third challenge is reconstruction of signals from separate electronic ADCs into a single output, that is, reconstruction of separate signals. This issue is not unique to photonic ADCs. Generally, this class of photonic ADC has exclusively used time-interleaved reconstruction. Selective Wavelength Interleaved Filtering Technique (SWIFT) reconstruction methods are based on hybrid filter channel bank (HFCB) methods operating in the frequency domain, capable of achieving near-perfect reconstruction of channelized input spectra. Applied to the ADC problem in the early 1990s, HFCB reconstruction is known to be less sensitive to interleaving errors than time domain approaches, particularly jitter related to high speed sampling. However, the gap between the performance of the electronic ADCs and the bandwidth of optical filters has hitherto been too great, making it untenable to apply frequency domain methods to the photonic ADC problem.
Some non-photonic attempts to solve the ADC problem have employed the HFCB technique in the electronic domain. However, they do not employ photonic filters or photonic local oscillator (LO) systems. Non-photonic solutions are subject to several limitations associated with the RF filtering and LO technology employed. For example, covering a very wide-band RF signal space is challenging because the filter technology changes as a function of frequency, and discrete LO signals are needed for each band.
Further, benefits of coherent optical frequency filtering for building channelizing RF filters have a long history. However, these efforts do not appear to have focused on the photonic ADC and by this omission exclude several key systems optimizations. Miniaturization was also not a focus. Finally, the combination of optical channel banks with HFCB reconstruction does not appear to have been addressed.
High resolution optical filters in the sub-GHz regime have been studied in Arrayed Waveguide Gratings (AWGs), silica-based ring resonators, and silicon micro-disk filters. Micro-structured resonators in general, while capable of sub-GHz response, are intrinsically infinite impulse response (IIR) elements, with strong phase distortion features. FIR response, being readily equalized, is therefore advantageous for HFCB applications. Echelle gratings, AWGs, and free space elements such as Virtually Imaged Phased Arrays (VIPA) have received study and reported GHz performance. All also beneficially provide a ‘self-registered’ characteristic in which filter passbands are registered to one another by virtue of the structure inherent to the filter and, beneficially, do not require individual alignment operations.
However, there is a need for technically superior ADC performance that overcomes the problems of existing systems. Further, there is a gap between ADCs and optical filter technology. Therefore, a need arises for technically superior ADC performance beyond the current state of the art, and an opportunity for photonically assisted ADC technology to meet this need, provided that several challenges can be overcome. Chief among these is a need for compatibility between optical filter technology and lower-rate ADCs.